The present invention relates to a pilot-controlled pressure limiting valve.
A conventional pressure limiting valve of this type is described, e.g., in data sheet RD25731/10.05 or data sheet RD 25818/08.03 from Bosch Rexroth AG. When an adjustable limiting pressure is reached, a main control slide or main control cone releases the connection between an inlet connection and a tank connection. The particular main control element—either a cone or slide—is acted upon in the opening direction by the inlet pressure on a control surface that faces the inlet connection. A control surface facing away from the inlet connection limits a control pressure space. A spring is located therein, which acts on the main control element in the closing direction. In addition, a control pressure is maintained in the control pressure space. The control pressure is adjustable using a pilot-controlled pressure limiting valve. The precontrol valve is supplied with control fluid via a throttle bore with a small cross section that is located in the center of the main control element. This bore connects the control pressure space with the inlet connection.
An important characteristic value of the pressure limiting valve is its constant-pressure behavior. In this regard, one considers the change in inlet pressure P with volumetric flow rate Q that passes through the pressure limiting valve and to the tank. The characteristic curve P(Q) increases due to the system.
A cause for this increase in the P(Q) characteristic curve are flow forces that occur at the main control element when it opens. High flow speeds in the outflowing pressure medium occur in the region around the annular opening surface. The static hydraulic pressure is therefore reduced at this point, per Bernoulli's equation. In the edge regions of the main control element, the control surface facing the inlet is therefore acted upon with a lower pressure than the inlet pressure. The equilibrium condition at the main control element—according to which the force of the spring and the force exerted by the control pressure on the main control element and the force exerted on the inlet-side control surface must be equal in order to hold the pressure limiting valve open—results in the increase in inlet pressure when the volumetric flow rate increases. As stated, this effect may be described using the image of flow forces that act in the direction of closing of the main control element, and which must be compensated for with a higher inlet pressure. It is therefore difficult to prevent a certain increase in the P(Q) characteristic curve. With poppet valves, the flow forces are even used to dampen the main control cone.
With the conventional pilot-controlled pressure limiting valves described above, the quantity of control fluid also increases when the inlet pressure increases. Due to further throttle points in the control fluid path to the precontrol valve, and given that the P(Q) characteristic curve of the precontrol valve also increases, the increase in the P(Q) characteristic curve of the precontrolled pressure limiting valve is amplified.